1. Field of the Invention
The present invention relates to a metal carrier for a catalytic converter that is formed as a honeycomb structure for use in a device for purifying exhaust gas in an internal combustion engine, and particularly to a metal carrier for a catalytic converter designed to prevent breaks or cracks due to thermal strain in a rolled honeycomb structure.
2. Description of the Related Art
As shown in FIG. 1, metal carriers for catalytic converters used in exhaust gas purifying systems of the prior art are constructed by interposing brazing filler metal between thin metal flat sheet 31 and thin metal corrugated sheet 32, stacking the flat sheet 31 and corrugated sheet 32, rolling the sheets into a rolled form from one edge to form a honeycomb structure 34, and using a high-vacuum furnace, melting the brazing filler metal to bond the contacting portions of the sheet material. A nickel-based brazing filler metal is used as the brazing filler metal, and ferritic stainless steel is used for the flat sheet and corrugated sheet. A honeycomb structure 34 formed in this way and housed within a metal casing 33 is known in the art as a metal carrier for a catalytic converter 30 (for example, Japanese Patent Laid-open 4373/81).
A catalyst-carrying layer composed of, for example, alumina, is formed on the surface of the honeycomb passages of the honeycomb structure, and a precious metal catalyst is carried on this catalyst-carrying layer which performs the function of exhaust gas-purifying catalysis. When installed in the path of exhaust from an internal combustion engine, such a construction purges HC, CO, and NO.sub.X contained within the exhaust gas. Out of the necessity to ensure a maximum of honeycomb passage surface area in a limited cubic volume, the flat and corrugated sheet are of the minimum thickness capable of maintaining strength.
In the metal carrier for a catalytic converter based on the above-described honeycomb structure of the prior art, the speed of flow of exhaust gas through the honeycomb structure is faster along the inner layers than along the outer layers, and accordingly, heat generation due to catalytic reaction and contact with hot exhaust gas, as well as heat radiation from the casing, result in a temperature distribution such that heat increases toward the inner layers and decreases toward the outer layers. Due to this heat distribution, expansion and contraction of the hotter inner layers of the honeycomb structure are greater than the expansion and contraction of the cooler outer layers of the honeycomb structure, and this variance results in thermal strain between the inner layers and outer layers. This thermal strain is repeated each time the honeycomb structure expands and contracts, and because all of the flat sheet 31 and corrugated sheet 32 is integrated as a single brazed structure by means of brazing filler metal as shown in FIG. 1, this strain cannot be released, resulting in the problem that over a long period of use, breaks are caused at the bonding points between the flat sheet 31 and corrugated sheet 32.